ifdef is a directive in the C and C++ programming languages that allows conditional compilation of code. The #ifdef directive tests whether a specific identifier has been defined previously in the file or via a command-line argument. If the identifier has been defined, the code block following the #ifdef directive is included for compilation; otherwise, it is excluded. This conditional compilation enables developers to create code that is included or excluded based on specific conditions, thereby customizing the behavior of the program during compilation. The #ifdef directive is often used in conjunction with #ifndef and #endif to create conditional compilation blocks that allow for greater flexibility in managing code for different environments or configurations.